All-wheel drive drivetrains for vehicles allow driving power to be delivered to all four wheels of the vehicle. By delivering driving power to all four wheels, vehicle performance is improved when surface conditions are poor. As examples, all-wheel drive systems can improve vehicle performance when road surfaces are slippery as a result of rain or snow, and when the road surface itself is of poor quality, such as dirt or gravel road surfaces.
In all wheel-drive vehicles with a transversely mounted front engine layout, a transaxle is typically utilized to provide a desired gear ratio and to distribute driving power to the left and right front wheels by way of a front differential and a front axle. A power transfer unit receives driving power from the front axle and transfers driving power to the rear wheels of the vehicle by way of a driveshaft, a rear differential, and a rear axle. In some implementations, a rotary coupling is provided to connect and disconnect power transfer to the rear wheels, and/or to prevent rotation of driveline components such as the drive shaft when power is not being supplied to the rear wheels. Some rotary couplings are also able to vary the amount of torque provided to the rear wheels of the vehicle.
One design for a rotary coupling uses a hydraulic clutch to connect and disconnect torque transmission between an input part and an output part. The terms “input part” and “output part” are used for convenience, and are not intended to imply that torque is always applied at the input part, as this can vary based on the configuration of a vehicle and the circumstances under which it is operated. When the clutch is engaged to transfer torque between the input part and the output part, hydraulic fluid within the clutch is utilized to lubricate and cool the clutch pack. When the clutch is disengaged, however, the presence of hydraulic fluid within the clutch pack can cause unintended partial engagement of the clutch, resulting in parasitic losses.